1. Field of the Invention
The present invention relates to a processing method for the creation of a slotted hole in a processed subject.
2. Description of the Related Art
In a vehicle such as a motor cycle, for example, in a vehicle body side bracket (a processed subject), as described in Japanese Unexamined Patent Publication No. 2007-302109 (Patent Document 1), right and left front forks are inserted to slotted holes provided in right and left sides of the vehicle body side bracket, and the front forks are clamped by fastening the slots by a split clamp bolt.
A conventional processing method for a slotted hole processes a hole 2 in a processed subject 1 such as a forged product, and processes a slot 3 intersecting the hole 2, as shown in FIG. 4. At this time, an original hole diameter d1 of the hole 2 caused by the hole processing of the processed subject 1 is reduced to a hole diameter d2 after the slot processing, due to a residual stress of the processed subject 1 caused by forging, a heat processing or the like. A tensile residual stress “a” (causing a contraction when released) is generated in an inner diameter portion of the hole 2 and a compressive residual stress “b” (causing an expansion when released) is generated in a periphery thereof, due to the hole processing of the processed subject 1. The tensile residual stress “a” released by the slot processing intersecting the hole 2 acts in a direction of contracting an inner peripheral portion of the hole 2. As a result, the hole diameter d1 of the hole 2 is reduced to the hole diameter d2. In the processed subject 1, if the hole diameter d2 of the hole 2 becomes smaller than an ideal dimension (a diameter close to the other subject), it becomes difficult to insert and assemble the other subject, and an obstacle is generated in an assembling step.
In this case, the following methods exist as a countermeasure for a diameter reduction after the slot processing of the slotted hole; however, all of them include a significant disadvantage.
(1) The original hole diameter d1 of the hole 2 formed by the hole processing of the processed subject 1 is processed larger in advance.
In this case, when the other subject is inserted to the hole 2 so as to be clamped, an inner diameter portion of the hole 2 is not well fitted to an outer diameter of the other subject, thereby causing a reduction of a slip load holding the other subject without relatively sliding and a reduction of a flexural fatigue strength in the clamped state of the other subject. In particular, if a high rigidity portion 1A having a partly high rigidity exists in the periphery of the hole 2 in the processed subject 1, deformation in the high rigidity portion 1A becomes small, deformation in a low rigidity portion 1B becomes large, and a curvature of the hole diameter d2 becomes small in the high rigidity portion 1A, and becomes large in the low rigidity portion 1B. As a result, the other subject comes to a condition that the other subject does not fit in the side of the high rigidity portion 1A.
(2) After the slot processing of the processed subject 1, upper and lower end surfaces around the hole 2 of the processed subject 1 are fixed from upper and lower sides, an inner diameter portion of the hole 2 is finished, and the hole diameter d2 is corrected to the ideal dimension.
In this case, since the processing number is increased, and the inner diameter portion of the hole 2 is processed in a state of being opened by the slot 3, it is hard to fix the processed subject 1, and there are disadvantages such as generation of fluttering, and reduction of processing dimension accuracy.